U.S. Pat. No. 4,914,412 relates to a specially designed magnetic circuit which, inter alia, comprises driving elements of a material with high-magnetostrictive properties, for example Terfenol. The magnetic material which is to be magnetized consists of a cylindrical body. It is surrounded by a magnetizing coil which has the same axial length as the cylindrical body. The two plane surfaces of a circle which are formed by the end surfaces of the body and the end surfaces of the coil are covered by relatively thin circular plates having the same outside diameters as the coil and are manufactured from a soft-magnetic material.
The magnetic circuit is magnetized statically by means of permanent magnets which are formed as discs having the same diameter as that of the soft-magnetic plates are arranged on top of these. The magnetic circuit finally comprises a cylindrical ring of ferromagnetic material having the same axial length as the coil and surrounding the coil.
Since as mentioned above the cylindrical body is made of high-magnetostrictive material, i.e. with a great linear expansion, the compressive stresses on the circular plates may become considerable. Since otherwise the magnetic circuit has a high dynamic performance, also fatigue problems may arise in connection with a high mechanical load.
From a magnetic point of view, it is important that the circular plates have as low reluctance as possible and that the eddy current and hysteresis losses are also as low as possible. One suitable material from these points of view is therefore, for example, a pressed powder mixture of soft-magnetic particles and plastic particles. It is then obvious that the mechanical loadability becomes restricted. If the material is subjected, for example, to a high compressive stress within the elastic range, this will result in a considerable elastic deformation, i.e. the compressive stress from the cylindrical body will be capable of being transmitted only to an insignificant extent to an external mechanical load. If the compressive stress results in a load on the material which exceeds the elastic range, the material will be pulverized. From a mechanical point of view, therefore, such a material is less suitable.